It is well established that the deterioration of alkaline calcareous masonry materials incorporated, for example, into buildings, engineering structures, public and funerary monuments and outdoor sculpture is associated with chemical weathering. For architectural and sculptural stone, the principal mechanism of chemical weathering is acidic dissolution of the carbonate minerals calcite and aragonite (calcium carbonate) and dolomite (calcium magnesium carbonate). In rural areas, the acidity of fog, rainwater and snow may be limited by the solubility of carbon dioxide. Considerably lower values of pH (representing greater acidity) are recorded in urban industrialized areas, where sulfur dioxide is more abundant, is readily oxidized by ozone and nitrogen oxides, and the resultant sulfur trioxide dissolves in water to form sulfuric acid. The macroscopic results of acidic attack include erosion of stone profiles and of surface tooling and carving, loss of polish, the progressive illegibility of inscriptions, and dramatic disintegration.
Much of the prior art with regard to protection of calcareous stone has relied upon the use of film-forming sealers, including those based upon drying oils, plant resins, shellac, waxes, and more recently (that is, since the Second World War) acrylics and epoxies. Total failure of such sealer-based systems via entrapment of moisture behind the film is common, as is the embrittlement, clouding and discoloration of the films upon environmental exposure to ultraviolet radiation. Similarly poor results have been reported with regard to the application of water-repellents (such as stearates, silicones and fluoropolymers), largely because of their inability to remain attached to the chemically dynamic carbonate minerals outdoors.
Where deterioration has advanced to the stage of significant reduction of cohesive properties, attempts have been made to utilize many of the organic treatments noted above as consolidants (strengtheners). For solution polymers, entry into the weakened pore structure of the stone is typically impeded by the tendency of solvent evaporation to limit deposition of the consolidant to a zone near the stone surface. With chemically curing resin systems (such as two-part epoxies), their high viscosity is a critical factor, resulting in insufficient penetration and, therefore, poor performance.
Progress in the consolidation of deteriorated masonry materials with commercial strengthening formulations based on tetraethyl orthosilicate (TEOS) has been substantial since the early 1970's. Hydrolysis and condensation of this low viscosity liquid (sometimes blended with alkyltrialkoxysilanes to impart water-repellency to the cured consolidant) results in the formation of glassy deposits of silica. Unfortunately, the results achieved for the preservation of sandstones, brick, and other silicate building materials have not been equaled for carbonate-containing rocks. In fact, carbonate minerals have recently been shown to be anti-catalytic to the formation of silicate polymers derived from TEOS. Use of TEOS-based products is claimed to result in improved cohesion of deteriorated stone as a result of the development of chemical bonds to the stone surface. It is firmly believed that such bond formation requires hydroxyl groups, which are largely absent in calcite, aragonite and dolomite.
Other methods historically proposed and/or utilized for the preservation of limestones and marbles include the reaction of the carbonate minerals with aqueous solutions of fluorides, fluorosilicates, and barium hydroxide. Published results of both laboratory and field testing of these formulations strongly suggest that they are of questionable value, especially for marble.
Since the 1960's, there has been a great deal of research on the response of calcium hydroxide (synthetic portlandite) in concrete and other cement-based composites to environmental acidity. The dissolution of portlandite weakens the microstructure of these materials. The calcium carbonate that typically forms as a result of this process (called carbonation) is, itself, sensitive to acidic conditions and is susceptible to further attack (sulfation).